A compressor used in a refrigerating cycle in which carbon dioxide is used as a coolant (CO2 cycle) needs to be designed with special care since the pressure and the temperature of the coolant discharged from the compressor in such a refrigerating cycle are bound to be higher than those in a refrigerating cycle in which a coolant such as R134a is used. At present, aluminum materials, which are lightweight and can be cast with ease, are most often used to constitute components (the housing and the internal mechanisms) of the compressor. However, since the tensile strength of aluminum becomes greatly reduced at high temperature, the wall thickness of a component, e.g., the housing, constituted of aluminum needs to be set to a significant value in design in order to assure a sufficient level of strength. For this reason, it is difficult to realize a compressor for a CO2 cycle as a compact unit.
The problem discussed above is addressed in a compressor in the related art used in an automotive air-conditioning system by forming the housing with an extremely sturdy material to enable miniaturization of the compressor (see Japanese Unexamined Patent Publication No. 2000-54958). In this publication of the invention in the related art, it is indicated that the elongation limit of the “sturdy material” should be equal to or higher than 500 N/mm2 and more desirably within a range of 700 to 800 N/mm2 (see Japanese Unexamined Patent Publication No. 2000-54958: paragraph 0012 and claims 7 and 8) and steel, bronze alloys, titanium and fiber-reinforced materials are listed as specific examples (see Japanese Unexamined Patent Publication No. 2000-54958: claims 2 to 6).
Alternatively, a compact compressor may be achieved by modifying the shapes of the components. In an example of this approach in the related art, the piston is formed in a staged shape that includes a large-diameter piston portion and a small-diameter piston portion so as to assume a staged shape and the cylinder bore is formed in the shape conforming to the external contour of the piston so as to reduce the Hertzian stress at the large-diameter piston portion and the large diameter bore portion, thereby achieving miniaturization of the compressor along the axial direction (see Japanese Unexamined Patent Publication No. H11-241677).
However, the “sturdy material” disclosed in Japanese Unexamined Patent Publication No. 2000-54958 described above is less than ideal for the following reasons. First, the materials cited in the publication do not have sufficient elongation limits (yield points) that will allow the compressor to be provided as a miniaturized and lightweight unit at low production costs while assuring the required component strength. In addition, steel, which is among the materials listed in the publication, cannot be cast and thus, the use of steel will lead to an increase in the molding cost. According to JIS H 5114, the minimum value of the tensile strength of a bronze alloy such as an aluminum bronze casting is equal to or smaller than 500 N/mm2, which is short of the required strength in the opinion on the inventor of the present invention et. al. Titanium is an expensive material and the tensile strength of pure titanium is equal to or less than 588 N/mm2 and is, therefore, not sufficient. Examples of fiber-reinforced materials include reinforced plastics. However, the tensile strength of such material is not high enough, e.g., 360 Nmm2 in the case of unsaturated polyester filled with high-strength fiberglass and 250 Nmm2 in the case of special nylon.
In addition, the invention disclosed in Japanese Unexamined Patent Publication No. H11-241677 does not directly relate to a structure for miniaturizing and reducing the weight of the housing which most affects the size and the weight of the entire compressor, and for this reason, it does not significantly contribute to miniaturization, weight reduction and cost reduction with regard to the compressor as a whole.
Accordingly, an object of the present invention is to provide an entire compressor as a miniaturized and lightweight unit at a lowered production cost by selecting an optimal material to constitute a component or by designing the housing in a specific shape so as to allow the component to have a smaller wall thickness while assuring sufficient strength.